The invention relates to a method for regulating the magnetic field delivered by a resistive magnet for eliminating any drifting of this field due more particularly to the temperature variations caused mainly by the Joule effect dissipation of the magnet itself; the invention also relates to a resistive magnet system using this method and, by way of application, an installation for forming images by nuclear magnetic resonance (NMR) incorporating such a system.
It is known that NMR image forming installations, among others, require a large sized magnet capable of generating a uniform magnetic field, called: basic field, in a given region in space. Typically, it is necessary to generate a field of 0.15 to 0.5 teslas with a homogeneity of 1 to 10 parts per million (ppm) in a sphere of a diameter of 50 cm at least.
Among the different types of magnets currently used for creating such fields, there may be mentioned the resistive magnets formed of one or more coils arranged, very often without a ferromagnetic shrouding, so as to obtain the required homogeneity. The stabilization of the operating point of such a magnet requires a considerable time for it is impossible to determine the value of the field in the center of the "sphere of homogeneity" because this volume is reserved for the patient and means for measuring the field cannot be installed therein and especially because, during operation, the magnetic field is disturbed by the subsidiary fields generated by the gradient coils of the NMR image forming installation.
The stability and uniformity of the magnetic field are then only really acquired when the temperature of the magnet has become completely stabilized, which does not exclude subsequent drifting, which is difficult to master. Because of all these difficulties, it has never been a question up to now of considering the value of the basic field as a parameter capable of being chosen by the operator in NMR image formation.
The purpose of the invention is to overcome all the disadvantages of the prior technique mentioned above.